rgwlc: permit disabling of (default) auto-clearing of stale sessions

rgwlc: permit disabling of (default) auto-clearing of stale sessions

Provide an option to disable automatic clearing of stale sessions--
which, unless disabled, happens after 2 lifecycle scheduling cycles.

The default behavior is most likely not desired when a debugging or
testing lifecycle processing with rgw_lc_debug_interval is set, and
therefore re-entering a running session after 2 scheduling cycles is
fairly likely.

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>